This invention relates to the well known general field of phosphate conversion coating of metals, particularly to the type of phosphate conversion coating known as iron side or non-crystalline conversion coating that is formed on predominantly ferriferous surfaces from a working phosphating composition that does not contain any substantial amount of any divalent cations that form insoluble phosphates, for example, zinc, nickel, cobalt, manganese, calcium, magnesium, or the like. For brevity hereinafter, this type of coating or type of composition for forming such a coating is called simply iron phosphate or a grammatical variation thereof.
In the automotive and industrial vehicle industry, metal parts are often cut and shaped prior to painting. In prior art processes for cutting metal, the cutting tool was typically a CNC machine or the like which cuts the metal by mechanical means, that is physical contact between the cutting tool and the workpiece to be cut. This method of cutting resulted in minimal scale or oxide being generated on the cut edge of the workpiece. The small amounts of scale and oxide enabled the manufacturer to phosphate and paint the workpiece after cutting with few paint adhesion problems, without additional steps to remove the scale and oxide. Thus, certain industries have developed processes and production lines that have no provision for removal of scale between the cutting stage and the phosphating stage.
With the introduction of laser forming and cutting of workpieces, a new problem of laser generated scale and oxide on the cut edge of workpieces arose. Manufacturers found that cutting ferrous metal with lasers resulted in sufficient scale and oxide formation at the cut edge of the workpiece to interfere with adhesion of subsequently applied paint layers and the corrosion resistance of the painted substrate. The poor paint adhesion caused appearance problems such as a “ribboning effect”, as well as susceptibility to corrosion that was unacceptable to the manufacturers. Conventional phosphating, typically zinc phosphating, previously used as a treatment when mechanical cutting means were used, did not improve the paint adhesion on the laser cut edges enough to meet manufacturing requirements.
One attempted solution to poor paint adhesion on laser cut edges was to pickle and oil the workpieces after cutting and prior to phosphating. However, this method required the addition of a new step and its attendant costs to the processing of the metal pieces. The additional step requires equipment, skilled workers as well as floor space or transport that had not been part of the economics of manufacturing the metal parts when mechanical cutting was used. Also, pickling solutions are typically strongly acidic solutions and can be hazardous for workers and the environment. Due to the nature of the chemicals involved in pickling, typically sulfuric acid or nitric acid, there are also environmental and waste disposal issues that must be addressed if a manufacturer chooses to use this method. Thus, particularly for manufacturers who had no need to pickle prior to the introduction of laser cutting, it is desirable to provide a treatment that enhances the adhesion of paint to the laser cut workpieces without the addition of new steps or significant extra costs.
Iron phosphating processes for treatment of metal are known in the art. Another attempted solution to the paint adhesion problems caused by scale and oxide generated by laser cutting was to replace the zinc phosphating treatment used prior to painting in the prior art process with an iron phosphating treatment. However, conventional iron phosphating treatments alone, typically having pHs of 3.0 or more and total acid points of 6 to 14 did not provide sufficient improvement to paint adhesion.